button click event does not fire when using form method="post"

<form id="form2" runat="server">
      <input type="hidden" runat="server" name="VPSProtocol" value="2.23" />
      <input type="hidden" runat="server" name="TxType" value="PAYMENT" />
      <input type="hidden" runat="server" name="Vendor" value="vendorname" />
      <input type="hidden" runat="server"name="Crypt" value="orderInfo" />
<asp:Button ID="btnTest" runat="server" Text="Test SagePay" runat="server" OnClick="onclick_click" />
      </form>

<script language="javascript" type="text/javascript">
        function PostDForm() {
            var form = document.createElement("form");
            form.setAttribute("method", "post");
            form.setAttribute("action", "test.aspx");

            var hiddenField = document.createElement("input");
            hiddenField.setAttribute("name", "idOfHiddenField");
            hiddenField.setAttribute("value", "valueOfHiddenField");
            form.appendChild(hiddenField);
            document.body.appendChild(form);
            form.submit();
        }
    </script>

protected void btn_Click(object sender, EventArgs e)
    {
        //Write code here to save your data to database

        ClientScript.RegisterStartupScript(this.GetType(),"keyToPostDFormFunction", "<script>PostDForm();</script>");
    }